There's not much you can do, but evaluate the effects to see if concrete cover is acceptable. You may be able to add some passive cathodic protection.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Sorry... misread the question.

I have seen this issue and a second pile was bored nearby. This is a reason not to like the auger-cast piles.

WMA

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

@Dik: Thank you for your response. I think CP system is an option to consider. What do you think about leaving the piles as they are if the structural engineers prove that these piles only receive compression loads? Or we can just add another shorter piles alongside to resist any uplift and lateral load effects as only the top portion of the piles need the reinforcing rebars?

@WMArmstrong: Thank you. Adding more piles is supposed to be the last resort.

It's not a strength issue, I think. It's a corrosion one. Unless you have fairly large bars, the flexural strength will not be diminished by much based on the cover. Corrosion would be my main issue.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

And even if compression only piles, some length of the pile would likely be ignored in terms of confinement of the concrete where it would need to act as a reinforced concrete column. You need the rebar for that.

Doing shorter piles also isn't a good option as those shorter piles will have different performance characteristics. When possible, uniformity in the foundation system is usually important. The project geotech would be a big help here, though, as they may be able to assist in finding an acceptable solution.

It depends on the loading and the pile... I've done hundreds of them with a single 15M or #5 bar in the center going into grade beams... for anchor rods, a small 2' 'cage'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Is the insufficient cover problem along the entire 33m length of the pile?
Is the soil aggressive for corrosion?
Check the capacity of the pile assuming reasonable corrosion reduction in the area of the as-built reinforcing steel?
Jet grout down along the perimeter of the pile?
Install a new pile?

www.PeirceEngineering.com